1. Field of the Invention
This invention relates to a radiation image processing apparatus for detecting radiation by a radiation detector and thereby acquiring a radiation image. This invention particularly relates to correction with respect to an artifact occurring in accordance with leak electric charges.
2. Description of the Related Art
For example, in the medical field, there have heretofore been widely used radiation image processing apparatuses, wherein radiation is irradiated from a radiation source toward an object (a patient), wherein the radiation carrying image information of the object is detected by a radiation detector, wherein the thus detected image information is processed, and wherein radiation image information of the object is thereby acquired. The thus acquired radiation image information is displayed on a display device and utilizing for making a diagnosis, and the like.
FIG. 2 is a schematic view showing a constitution of a radiation detector employed in a radiation image processing apparatus. With reference to FIG. 2, a radiation detector 2 is provided with a plurality of detection elements 11 to 55, which are arrayed in a matrix-like form. Each of the plurality of the detection elements 11 to 55 comprises a thin film transistor (TFT) acting as a signal readout switch. Also, a film of amorphous selenium (a-Se), or the like, is formed by vacuum evaporation on the detection elements 11 to 55.
Gate lines 4a to 4e, each of which is selected by a gate driver circuit 3, and signal lines 6a to 6e, each of which is selected by a signal readout circuit 5, are connected to the corresponding detection elements among the detection elements 11 to 55. In accordance with control signals given by a readout control circuit 7, one of the gate lines 4a to 4e is selected by the gate driver circuit 3, and one of the signal lines 6a to 6e is selected by the signal readout circuit 5. One of the detection elements 11 to 55 is thus specified, and electric charge information having been accumulated in the specified detection element is thus read out.
FIG. 3 shows an equivalent circuit of each of the detection elements 11 to 55. Each of the detection elements 11 to 55 comprises a corresponding region of a common electrode 8, to which a bias voltage is applied from an electric power source B. Each of the detection elements 11 to 55 also comprises a corresponding region of a conversion layer 9 for converting radiation X into electric charge signal component. The conversion layer 9 is constituted of a-Se, or the like. Each of the detection elements 11 to 55 further comprises a pixel electrode 10 for collecting electric charges generated in the corresponding region of the conversion layer 9. Each of the detection elements 11 to 55 still further comprises a storage capacitor C for storing the electric charges having been collected by the pixel electrode 10. Each of the detection elements 11 to 55 also comprises a transistor switch Tr for outputting the electric charge information, which has been stored by the storage capacitor C, to an external circuit. The storage capacitor C is connected to a source terminal of the transistor switch Tr. Also, a corresponding gate line among the gate lines 4a to 4e is connected to a gate terminal of the transistor switch Tr. Further, a corresponding signal line among the signal lines 6a to 6e is connected to a drain terminal of the transistor switch Tr.
In cases where an excessive dose of the radiation X is irradiated to the radiation detector 2, and a large quantity of the electric charges are stored in the storage capacitor C at the time at which the transistor switch Tr is in the off state, there is the risk that the voltage across the drain terminal and the source terminal will increase excessively, and that the transistor switch Tr will thus be broken down.
Therefore, the radiation detector 2 is set so as to apply the bias voltage of a negative voltage from the electric power source B to the common electrode 8, such that the polarity (negative voltage) identical with the direction of transition of a control voltage at the time, at which the transistor switch Tr is set from the on state to the off state, is set. In such cases, at the time, at which a voltage Vs of the storage capacitor C has fallen toward the negative voltage side as a result of the irradiation of the radiation X and has become lower than a gate voltage Vg having been set at a negative voltage, the transistor switch Tr is forcibly turned on, and the electric charges having been stored in the storage capacitor C are discharged. As a result, the breakdown of the transistor switch Tr is avoided.
In cases where the radiation detector 2 is constituted in the manner described above, if the period of time having elapsed between the finish of the irradiation of the radiation X and the beginning of the readout is short at the time at which the electric charge information is read out from each of the detection elements 11 to 55 by use of the signal readout circuit 5, the problems described below will occur. Specifically, at the time at which the transistor switch Tr is forcibly turned on due to the irradiation of an excessive dose of the radiation X to the radiation detector 2, an inappropriate artifact arises in the acquired radiation image due to adverse effects of leak electric charges discharged from each of the detection elements 11 to 55.
In order to eliminate the problems described above, the applicant proposed, in U.S. Pat. No. 7,822,179, an apparatus enabling the acquisition of an appropriate radiation image signal free from an artifact due to leak electric charges. The proposed apparatus comprises: (a) acquiring both of on control electric charge information (a signal value representing electric charge information and leak electric charges superimposed one upon the other), which is outputted at the time of on control, and off control electric charge information (a signal value representing the leak electric charges alone), which is outputted at the time of off control, with respect to each of the gate lines 4a to 4e at the time at which the electric charge information is read out from each of the detection elements 11 to 55 in the order of the gate lines 4a to 4e, and (b) making correction by subtracting the off control electric charge information, which represents the leak electric charges obtained at the time of the off control of each of the detection elements 11 to 55, from the on control electric charge information having been read out from the corresponding detection element.
However, with the apparatus proposed in U.S. Pat. No. 7,822,179, in order for the artifact due to the leak electric charges to be corrected, it is necessary to acquire the two kinds of the signal components, i.e. the on control electric charge information and the off control electric charge information, with respect to each of the detection elements. Therefore, the required signal readout time becomes two times as long as the signal readout time which is required in cases where the correction of the artifact is not performed. Accordingly, at the time of the imaging operations for acquiring a dynamic image, the problems occur in that the frame rate becomes low. Also, at the time of tomosynthesis imaging operations, the problems occur in that the period of time required before the imaging operations are completed, i.e. the period of time during which the patient is constrained for the imaging operations, becomes long.
Therefore, there is a strong demand for an apparatus, which enables a radiation image signal to be acquired quickly in cases where the artifact due to the leak electric charges is corrected.